Offsetting a turn horizontally

[Joseph Wakeling]

Hello all,

I'm using Lilypond 2.10.25 to typeset a piece (one of the Rose 32
clarinet etudes) that incorporates a turn on the _second beat_ of a
minim (half-note).

Here's the Lilypond code of the relevant segment as I have it now:

\relative c' {
c4.(\p e8 g c d \setTextCresc e)\<
\once \override Script #'extra-offset = #'(2.5 . 0) f2(\!\turn g4 f)
}

The trouble is that the extra-offset element moves the turn a constant
distance on the page, whereas I would like it to be offset by a
_musical_ distance---that is, reliably falling on the second beat.

I wonder if there's any way of achieving this?

Many thanks,

    -- Joe